U.S. Pat. No. 4,975,354 issued Dec. 4, 1990, entitled "Photographic Element Comprising An Ethyleneoxy-Substituted Amino Compound And Process Adapted To Provide High Contrast Development", by Harold I. Machonkin and Donald L. Kerr, describes silver halide photographic elements having incorporated therein a hydrazine compound which functions as a nucleator and an amino compound which functions as an incorporated booster. Such elements provide a highly desirable combination of high photographic speed, very high contrast and excellent dot quality, which renders them very useful in the field of graphic arts. Moreover, since they incorporate the booster in the photographic element, rather than using a developing solution containing a booster, they have the further advantage that they are processable in conventional, low cost, rapid-access developers.
A photographic system depending on the conjoint action of hydrazine compounds which function as nucleators and amino compounds which function as boosters is an exceedingly complex system. It is influenced by both the composition and concentration of the nucleator and the booster and by many other factors including the pH and composition of the developer and the time and temperature of development. The goals of such a system include the provision of enhanced speed and contrast, together with excellent dot quality and low pepper fog.
The goal of achieving low pepper fog is one which is exceptionally difficult to achieve without sacrificing other desired properties such as speed and contrast. (The term "pepper fog" is commonly utilized in the photographic art, and refers to fog of a type characterized by numerous fine black specks). A particularly important film property is "discrimination", a term which is used to describe the ratio of the extent of shoulder development to pepper fog level. Good discrimination, i.e., full shoulder development with low pepper fog, is necessary to obtain good halftone dot quality.
The use of ionizable thiones as antifoggants and/or stabilizers in nucleated high contrast films is disclosed in U.S. Pat. No. 4,166,742. The use of tetraazaindenes in nucleated high contrast films for such purposes as increasing sensitivity, enhancing emulsion stability, reducing fog formation and restraining grain growth is disclosed in numerous patents such as, for example, U.S. Pat. Nos. 4,241,164, 4,311,781, 4,912,017, 4,920,034, 4,939,067 and 4,978,603. Compounds which reduce pepper fog, however, often have an undesirably strong restraining action which leads to substantial undesired speed losses along with the desired pepper fog reduction.
The use of 2-substituted-4-hydroxy-1,3,3a,7-tetraazaindenes, in which the substituent at the 2-position is alkylthio or cycloalkylthio, as image toners in photographic materials is disclosed in U.S. Pat. No. 4,728,601. However, toning action is not related to ability to inhibit pepper fog.
The present invention is directed toward the objective of providing novel high contrast silver halide photographic elements which exhibit improved characteristics in regard to control of pepper fog, while still retaining excellent characteristics with respect to speed, contrast and full shoulder development.